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[[Image:Nuclear power defense in depth.png|280px|thumb|This diagram
demonstrates defense-in-depth in modern nuclear power plants. Current plants
may have some or all of these defenses, the defenses vary depending
on the type of plant, the nation constructing them, the use
(civilian, military, naval vessels) and the generation the plant is
from.

5th layer is the reactor
building or in newer powerplants a second outer containment
building.]]

Nuclear safety covers the actions taken to prevent
nuclear and radiation
accidents or to limit their consequences. This covers nuclear power plants as well as all
other nuclear facilities, the transportation of nuclear materials,
the use and storage of nuclear materials for medical, power,
industry, and military uses. In addition, there are safety issues
involved in products created with radioactive materials. Some of
the products are legacy ones (such as watch faces), others, like smoke detectors, are
still being produced.

Nuclear weapon safety, as well as the
safety of military research involving nuclear materials, is
generally handled by separate agencies than civilian safety, for
various reasons, including secrecy.

Agencies

Many nations utilizing nuclear power
have special institutions overseeing and regulating nuclear
safety.

Complexity

Nuclear power plants are some of the most sophisticated and complex
energy systems ever designed, and critics have seen nuclear power
as a dangerous, expensive way to boil water to generate
electricity. Proponents have argued that much of that complexity is
due to redundancy of systems, extensive backups, and the defense in depth strategy of design.
However, any complex system, no matter how well it is designed and
engineered, cannot be deemed failure-proof. This is especially true
if people are required to operate controls that dictate how the
system functions. Stephanie Cooke
has reported that:

The reactors themselves were enormously complex
machines with an incalculable number of things that could go wrong.
When that
happened at Three Mile Island in 1979, another fault line in the nuclear world
was exposed. One malfunction led to another, and then to a
series of others, until the core of the reactor itself began to
melt, and even the world's most highly trained nuclear engineers
did not know how to respond. The accident revealed serious
deficiencies in a system that was meant to protect public health
and safety.

A fundamental issue related to complexity is that the nuclear power
systems have an exceedingly long stay time. The timeframe involved
from the start of construction of a commercial nuclear power
station, through to the safe disposal of its last radioactive
waste, may be 100-150 years.

Failure modes of nuclear powerplants

There are concerns that a combination of human and mechanical error
at a nuclear facility could result significant harm to people and
the environment:

Operating nuclear reactors contain large amounts of
radioactive fission products which, if dispersed, could pose a
direct radiation hazard, contaminate soil and vegetation, and be
ingested by humans and animals. Human exposure at high enough
levels can cause both short-term illness and death, and longer-term
deaths by cancer and other diseases.

Nuclear reactors can fail in a variety of ways. Should the
instability of the nuclear material generate unexpected behavior,
it may result in an uncontrolled power excursion.
Normally, the cooling system in a reactor is designed to be able to
handle the excess heat this causes, however, should the reactor
also experience a loss-of-coolant
accident, then the fuel may melt, or cause the vessel it is
contained in to overheat and melt. This event is called a nuclear meltdown. Because the heat
generated can be tremendous, immense pressure can build up in the
reactor vessel, resulting in a steam
explosion such as happened at Chernobyl.

Hazards of nuclear material

Nuclear material and materiel may be hazardous if not properly
handled or disposed of. Experiments of near critical mass sized pieces of nuclear material
can pose a risk of a criticality
accident. David Hahn serves as an
excellent example of a nuclear experimenter who failed to develop
or follow proper safety protocols. Such failures raise the specter
of radioactive
contamination.

Even when properly contained, fission by-products which are no
longer useful generate radioactive
waste, which must be properly disposed of. In addition,
material exposed to neutron
radiation — present in nuclear reactors — may become
radioactive in its own right, or become contaminated with nuclear
waste. Additionally, toxic or dangerous chemicals may be used as
part of the plant's operation, which must be properly handled and
disposed of.

Vulnerability of plants to attack

Nuclear power plants are generally (although not always) considered
"hard" targets. In the US, plants are surrounded by a double row of
tall fences which are electronically monitored. The plant grounds
are patrolled by a sizeable force of armed guards. The NRC's
"Design Basis Threat" criteria for plants is a secret, and so what
size attacking force the plants are able to protect against is
unknown. However, to scram a plant takes less
than 5 seconds while unimpeded restart takes hours, severely
hampering a terrorist force in a goal to release
radioactivity.

Attack from the air is a more problematic concern. The most
important barrier against the release of radioactivity in the event
of an aircraft strike is the containment building and its missile
shield. Current NRC Chairman Dale Klein has said "Nuclear power
plants are inherently robust structures that our studies show
provide adequate protection in a hypothetical attack by an
airplane. The NRC has also taken actions that require nuclear power
plant operators to be able to manage large fires or explosions—no
matter what has caused them."

In addition, supporters point to large studies carried out by the
US Electric Power Research Institute that tested the robustness of
both reactor and waste fuel storage, and found that they should be
able to sustain a terrorist attack comparable to the September 11 terrorist attacks in
the USA. Spent fuel is usually housed inside the plant's "protected
zone" or a spent
nuclear fuel shipping cask; stealing it for use in a "dirty bomb" is extremely difficult. Exposure to
the intense radiation would almost certainly quickly incapacitate
or kill anyone who attempts to do so.

The Brookhaven Report: Theoretical Possibilities and
Consequences of Major Accidents in Large Nuclear Power PlantsWASH-740 1957

The AP1000 has a maximum core damage frequency of 5.09 x
10-7 per plant per year. The Evolutionary Power Reactor (EPR)
has a maximum core damage frequency of 4 x 10-7 per
plant per year. General Electric has recalculated maximum core
damage frequencies per year per plant for its nuclear power plant
designs: